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Cardiac BIN1 folds T-tubule membrane, controlling ion flux and limiting arrhythmia.

Hong T, Yang H, Zhang SS, Cho HC, Kalashnikova M, Sun B, Zhang H, Bhargava A, Grabe M, Olgin J, Gorelik J, Marbán E, Jan LY, Shaw RM - Nat. Med. (2014)

Bottom Line: Bridging integrator 1 (BIN1) is a T-tubule protein associated with calcium channel trafficking that is downregulated in failing hearts.We also found that T-tubule inner folds are rescued by expression of the BIN1 isoform BIN1+13+17, which promotes N-WASP-dependent actin polymerization to stabilize the T-tubule membrane at cardiac Z discs.When the amount of the BIN1+13+17 isoform is decreased, as occurs in acquired cardiomyopathy, T-tubule morphology is altered, and arrhythmia can result.

View Article: PubMed Central - PubMed

Affiliation: 1] Cedars-Sinai Heart Institute, Cedars-Sinai Medical Center, Los Angeles, California, USA. [2].

ABSTRACT
Cardiomyocyte T tubules are important for regulating ion flux. Bridging integrator 1 (BIN1) is a T-tubule protein associated with calcium channel trafficking that is downregulated in failing hearts. Here we find that cardiac T tubules normally contain dense protective inner membrane folds that are formed by a cardiac isoform of BIN1. In mice with cardiac Bin1 deletion, T-tubule folding is decreased, which does not change overall cardiomyocyte morphology but leads to free diffusion of local extracellular calcium and potassium ions, prolonging action-potential duration and increasing susceptibility to ventricular arrhythmias. We also found that T-tubule inner folds are rescued by expression of the BIN1 isoform BIN1+13+17, which promotes N-WASP-dependent actin polymerization to stabilize the T-tubule membrane at cardiac Z discs. BIN1+13+17 recruits actin to fold the T-tubule membrane, creating a 'fuzzy space' that protectively restricts ion flux. When the amount of the BIN1+13+17 isoform is decreased, as occurs in acquired cardiomyopathy, T-tubule morphology is altered, and arrhythmia can result.

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Bin1 deletion increases extracellular K+ diffusion, prolonging action potential duration and increasing ventricular ectopy. (a) Representative patch clamp recording of IK1 current changes when quickly switching extracellular potassium concentration in a wildtype (WT) cardiomyocyte. (b) Kinetics of IK1 during K+on in WT and Bin1 HT cardiomyocytes (dotted line, dead volume time of 124 ms). (c) Comparison of the initial delay X0 of K+on for WT (n = 20) and Bin1 HT (n = 19) cardiomyocytes (P = 0.0045). (d) Kinetics of IK1 during K+off (1−∆IK1) in WT and Bin1 HT cardiomyocytes. (e) Comparison of X0 of K+off for WT (n = 20) and Bin1 HT (n = 19) cardiomyocytes (P = 0.0018). (f) Top: representative tracings of EKG (top) and TMP (transmembrane potential, bottom) from isolated and langendorff perfused WT (left) and Bin1 HT (right) hearts. Bottom: Action potential duration (APD80) is always prolonged in Bin1 HT hearts whether subjected to low (2.5 mM), normal (5 mM), and high (8 mM) potassium solution (left), and ventricular ectopy is increased in Bin1 HT hearts (right, incidence of arrhythmias during physiological buffer perfusion). (g) Ventricular activation map (left) and conduction velocity (right) of WT and Bin1 HT hearts subjected to high potassium (8 mM) perfusion (*, P < 0.05). Data are presented as mean ± SEM and cardiomyocytes are from three mice for each genotype, student’s t-test was used for statistical analysis.
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Figure 3: Bin1 deletion increases extracellular K+ diffusion, prolonging action potential duration and increasing ventricular ectopy. (a) Representative patch clamp recording of IK1 current changes when quickly switching extracellular potassium concentration in a wildtype (WT) cardiomyocyte. (b) Kinetics of IK1 during K+on in WT and Bin1 HT cardiomyocytes (dotted line, dead volume time of 124 ms). (c) Comparison of the initial delay X0 of K+on for WT (n = 20) and Bin1 HT (n = 19) cardiomyocytes (P = 0.0045). (d) Kinetics of IK1 during K+off (1−∆IK1) in WT and Bin1 HT cardiomyocytes. (e) Comparison of X0 of K+off for WT (n = 20) and Bin1 HT (n = 19) cardiomyocytes (P = 0.0018). (f) Top: representative tracings of EKG (top) and TMP (transmembrane potential, bottom) from isolated and langendorff perfused WT (left) and Bin1 HT (right) hearts. Bottom: Action potential duration (APD80) is always prolonged in Bin1 HT hearts whether subjected to low (2.5 mM), normal (5 mM), and high (8 mM) potassium solution (left), and ventricular ectopy is increased in Bin1 HT hearts (right, incidence of arrhythmias during physiological buffer perfusion). (g) Ventricular activation map (left) and conduction velocity (right) of WT and Bin1 HT hearts subjected to high potassium (8 mM) perfusion (*, P < 0.05). Data are presented as mean ± SEM and cardiomyocytes are from three mice for each genotype, student’s t-test was used for statistical analysis.

Mentions: Next, we asked whether the physical barrier formed by BIN1-organized membrane folds applies to other ions. With regard to potassium ions, which can also be affected by slow diffusion8, we measured current from the inwardly rectifying potassium channel, IK1, which maintains the cardiomyocyte resting membrane potential. As indicated in Fig. 3a, steady-state IK1 and its kinetics were recorded following rapid wash-in and wash-out of K+ in T-tubules. Similar to our observations of calcium diffusion, decay of IK1 in response to an increase of extracellular potassium concentration from 5 mM to 8.1 mM (K+on) is significantly faster in Bin1 HT cardiomyocytes (Fig. 3b, c). The smaller difference of K+ diffusion (24 ms versus 64 ms for calcium) between WT and Bin1 HT likely reflects less T-tubule based enrichment of IK128,29. Similarly, IK1 response to decreases of extracellular potassium concentration from 8.1 mM back to 5 mM (K+off) is also significantly faster in Bin1 HT cardiomyocytes (Fig. 3d, e).


Cardiac BIN1 folds T-tubule membrane, controlling ion flux and limiting arrhythmia.

Hong T, Yang H, Zhang SS, Cho HC, Kalashnikova M, Sun B, Zhang H, Bhargava A, Grabe M, Olgin J, Gorelik J, Marbán E, Jan LY, Shaw RM - Nat. Med. (2014)

Bin1 deletion increases extracellular K+ diffusion, prolonging action potential duration and increasing ventricular ectopy. (a) Representative patch clamp recording of IK1 current changes when quickly switching extracellular potassium concentration in a wildtype (WT) cardiomyocyte. (b) Kinetics of IK1 during K+on in WT and Bin1 HT cardiomyocytes (dotted line, dead volume time of 124 ms). (c) Comparison of the initial delay X0 of K+on for WT (n = 20) and Bin1 HT (n = 19) cardiomyocytes (P = 0.0045). (d) Kinetics of IK1 during K+off (1−∆IK1) in WT and Bin1 HT cardiomyocytes. (e) Comparison of X0 of K+off for WT (n = 20) and Bin1 HT (n = 19) cardiomyocytes (P = 0.0018). (f) Top: representative tracings of EKG (top) and TMP (transmembrane potential, bottom) from isolated and langendorff perfused WT (left) and Bin1 HT (right) hearts. Bottom: Action potential duration (APD80) is always prolonged in Bin1 HT hearts whether subjected to low (2.5 mM), normal (5 mM), and high (8 mM) potassium solution (left), and ventricular ectopy is increased in Bin1 HT hearts (right, incidence of arrhythmias during physiological buffer perfusion). (g) Ventricular activation map (left) and conduction velocity (right) of WT and Bin1 HT hearts subjected to high potassium (8 mM) perfusion (*, P < 0.05). Data are presented as mean ± SEM and cardiomyocytes are from three mice for each genotype, student’s t-test was used for statistical analysis.
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Related In: Results  -  Collection

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Figure 3: Bin1 deletion increases extracellular K+ diffusion, prolonging action potential duration and increasing ventricular ectopy. (a) Representative patch clamp recording of IK1 current changes when quickly switching extracellular potassium concentration in a wildtype (WT) cardiomyocyte. (b) Kinetics of IK1 during K+on in WT and Bin1 HT cardiomyocytes (dotted line, dead volume time of 124 ms). (c) Comparison of the initial delay X0 of K+on for WT (n = 20) and Bin1 HT (n = 19) cardiomyocytes (P = 0.0045). (d) Kinetics of IK1 during K+off (1−∆IK1) in WT and Bin1 HT cardiomyocytes. (e) Comparison of X0 of K+off for WT (n = 20) and Bin1 HT (n = 19) cardiomyocytes (P = 0.0018). (f) Top: representative tracings of EKG (top) and TMP (transmembrane potential, bottom) from isolated and langendorff perfused WT (left) and Bin1 HT (right) hearts. Bottom: Action potential duration (APD80) is always prolonged in Bin1 HT hearts whether subjected to low (2.5 mM), normal (5 mM), and high (8 mM) potassium solution (left), and ventricular ectopy is increased in Bin1 HT hearts (right, incidence of arrhythmias during physiological buffer perfusion). (g) Ventricular activation map (left) and conduction velocity (right) of WT and Bin1 HT hearts subjected to high potassium (8 mM) perfusion (*, P < 0.05). Data are presented as mean ± SEM and cardiomyocytes are from three mice for each genotype, student’s t-test was used for statistical analysis.
Mentions: Next, we asked whether the physical barrier formed by BIN1-organized membrane folds applies to other ions. With regard to potassium ions, which can also be affected by slow diffusion8, we measured current from the inwardly rectifying potassium channel, IK1, which maintains the cardiomyocyte resting membrane potential. As indicated in Fig. 3a, steady-state IK1 and its kinetics were recorded following rapid wash-in and wash-out of K+ in T-tubules. Similar to our observations of calcium diffusion, decay of IK1 in response to an increase of extracellular potassium concentration from 5 mM to 8.1 mM (K+on) is significantly faster in Bin1 HT cardiomyocytes (Fig. 3b, c). The smaller difference of K+ diffusion (24 ms versus 64 ms for calcium) between WT and Bin1 HT likely reflects less T-tubule based enrichment of IK128,29. Similarly, IK1 response to decreases of extracellular potassium concentration from 8.1 mM back to 5 mM (K+off) is also significantly faster in Bin1 HT cardiomyocytes (Fig. 3d, e).

Bottom Line: Bridging integrator 1 (BIN1) is a T-tubule protein associated with calcium channel trafficking that is downregulated in failing hearts.We also found that T-tubule inner folds are rescued by expression of the BIN1 isoform BIN1+13+17, which promotes N-WASP-dependent actin polymerization to stabilize the T-tubule membrane at cardiac Z discs.When the amount of the BIN1+13+17 isoform is decreased, as occurs in acquired cardiomyopathy, T-tubule morphology is altered, and arrhythmia can result.

View Article: PubMed Central - PubMed

Affiliation: 1] Cedars-Sinai Heart Institute, Cedars-Sinai Medical Center, Los Angeles, California, USA. [2].

ABSTRACT
Cardiomyocyte T tubules are important for regulating ion flux. Bridging integrator 1 (BIN1) is a T-tubule protein associated with calcium channel trafficking that is downregulated in failing hearts. Here we find that cardiac T tubules normally contain dense protective inner membrane folds that are formed by a cardiac isoform of BIN1. In mice with cardiac Bin1 deletion, T-tubule folding is decreased, which does not change overall cardiomyocyte morphology but leads to free diffusion of local extracellular calcium and potassium ions, prolonging action-potential duration and increasing susceptibility to ventricular arrhythmias. We also found that T-tubule inner folds are rescued by expression of the BIN1 isoform BIN1+13+17, which promotes N-WASP-dependent actin polymerization to stabilize the T-tubule membrane at cardiac Z discs. BIN1+13+17 recruits actin to fold the T-tubule membrane, creating a 'fuzzy space' that protectively restricts ion flux. When the amount of the BIN1+13+17 isoform is decreased, as occurs in acquired cardiomyopathy, T-tubule morphology is altered, and arrhythmia can result.

Show MeSH
Related in: MedlinePlus